It is well known that a sparkless operating region or band of a DC dynamoelectric machine incorporating interpoles therein shifts toward an excitation reducing side in accordance with its rotating speed.
Japanese Patent Application Laid-Open No. 62-71463 (1987) discloses a DC dynamoelectric machine incorporating, interpoles therein and having itself provisions therein for suppressing the shift of the sparkless operating region, in that, an adjusting magnetic member is disposed between the respective main poles and interpoles, optionally through air gap therebetween, so as to control leakage of interpole magnetic flux in such a manner that the leakage is restricted during a low speed operation and is increased during a high speed operation. However such DC dynamoelectric machine is found out to be insufficient to suppress the shift of the sparkless operation region and to maintain the, sparkless operation during transient operations, i.e., during a sudden change of load current during and over load operations up to 200% of rated load current which are frequently encountered during the driving of the cold rolling mill.